FIG. 23 includes explanatory views showing the display states in the power-on state and in the power-off state of a common conventional display device. As shown in FIG. 23, a display device 21 in the power-on state shows an image in a display region A. A region (frame region B) called a frame or a bezel in the periphery of the display region A does not contribute to image display. In contrast, a display device 22 in the power-off state shows no image in the display region A, and the frame region B still does not contribute to image display.
Such a common conventional display device is proposed to achieve image display with a high luminance in a display state such as a power-on state when combined with, for example, a polarized-light-diffusing film (e.g. Patent Literature 1). In a non-display state such as a power-off state, such a conventional display device unfortunately shows only a black or gray screen so that the device is useless for users. Besides, a large-size, difficult-to-move display device (e.g. digital signage and television receivers) can be merely an obstruction for users because it occupies the same space either in the non-display state or in the display state. Especially, a black screen that is shown by a conventional display device placed in a bright room fails to match the bright-color-based interior or wall (wall 23 in FIG. 23) or the housing of the display device and thus may cause uncomfortable feeling. In summary, common conventional display devices have their own value only when they are in the display state.
In order to solve these disadvantages, a mirror display is proposed which includes a half mirror plate including a half mirror layer on the viewing surface side of a display device so that it can serve as a mirror in the non-display state (e.g. Patent Literatures 2 to 5). Such a mirror display can serve not only as a display, which is the original purpose, but also as a mirror. Specifically, when display light is emitted from the display device, the mirror display shows an image owing to the display light in a region where the display light is emitted from the display device. When no display light is emitted from the display device, on the other hand, the mirror display reflects the outside light and serves as a mirror in a region where the display light is not emitted from the display device.
The half mirror layer is made of an optical member with a reflective function. Examples of the optical member include reflective polarizers such as multi-layer reflective polarizers and wire-grid reflective polarizers (e.g. Patent Literatures 6 and 7). A reflective polarizer reflects a polarized light component parallel to the reflection axis and transmits a polarized light component perpendicular to the reflection axis, among the incident light components. The reflective polarizer thus can transmit the light emitted from the display device to the viewing surface side as display light and can reflect outside light perpendicular to the polarization direction of the display light to the viewing surface side. A mirror display that includes a reflective polarizer as a half mirror layer utilizes such principles to switch the display mode and the mirror mode.